The benefits of optical fibres have been demonstrated in a number of sensing applications. The two major areas are: (i) distributed optical fibre sensors, and (ii) multiplexed point sensor arrays.
Distributed sensors utilise the intensity of backscatter light, with Raman and/or Brillouin peaks in the light signal utilised to measure temperature, strain or pressure. Distributed sensors offer a number of advantages including continuous sensing along the entire length of fibre, and flexibility and simplicity of the sensor, which may be standard telecoms optical fibre. For example, a distributed sensor may provide 10,000 measurement points along 10 km of optical fibre with a 1 m spatial resolution. Distributed sensor systems therefore offer low installation and ownership costs.
However, due to their slow response, distributed sensors are usually only used in applications where measurements taking in order of several seconds to hours are acceptable. The most common sensors of this type are the distributed temperature sensors (DTS), which are made by a number of companies. A typical performance of a DTS is 1 m spatial resolution and 1° C. temperature resolution in 60 seconds over a 10 km range.
Distributed sensors have also been used to measure strain by utilising Brillouin shifts in reflected or backscattered light, as described in U.S. Pat. No. 6,555,807 [1] or WO 98/27406 [2]. The frequency of the Brillouin shift is about 1 MHz/10με and its linewidth is about 30 MHz. The strain in an order of 10με can be determined along an optical fibre using the narrow frequency scanning methods described. However, using these approaches, the scanning rate is much slower than the pulse repetition rate and measurement times are typically in the order of few seconds to few minutes.
More recently, a technique for faster measurement of Brillouin frequency shift has been proposed in U.S. Pat. No. 7,355,163 [3]. This technique uses a frequency to amplitude convertor which may be in a form of an optical fibre Mach-Zehnder interferometer with a 3×3 coupler at its output. However, the strain resolution is limited by the linewidth of the Brillouin light and therefore the optical path length difference in the interferometer should be kept within the coherence length of the Brillouin light. Also, the polarisation fading between the two paths of the interferometer, the offset and gain variations of the photodetector receivers would significantly limit the strain measurement. Measurement times of around 0.1 seconds (10 Hz) with strain resolution of 50με have been recently reported using this technique.
For many applications, such as acoustic sensing, much higher sensitivities and faster a measurement time in the order of 1 millisecond (1 kHz), 0.1 millisecond (10 kHz) or 0.01 millisecond (100 kHz) is required.
Multiplexed point sensors offer fast measurements with high sensitivity and are used, for example, in hydrophone arrays. The main application for these in the energy market is for towed and seafloor seismic arrays. However, unlike with distributed sensors, multiplexed point sensors cannot be used where full coverage is required. The size and the position of the sensing elements are fixed and the number of sensors multiplexed on a single fibre is typically limited to 50 to 100 elements. Furthermore, the sensor design relies on additional optical fibre components leading to bulky and expensive array architectures. There is also considerable effort to increase the number of sensors that can be efficiently multiplexed on a single length of fibre.
Optical-time-domain reflectometry (OTDR) is a well known technique that has been used to test optical fibre communications cables. In order to reduce the effect of coherent backscatter interference, which is sometime is referred to as Coherent Rayleigh Noise, a broadband light source is normally used. However, proposals have also been made in U.S. Pat. No. 5,194,847 [4] to use coherent OTDR for sensing intrusion by detecting the fast changes in a coherent backscatter Rayleigh signal. In addition, Shatalin et al. [5] describes using coherent Rayleigh as a distributed optical fibre alarm sensor.
WO 2008/056143 [6] describes a disturbance sensor similar to that of U.S. Pat. No. 5,194,847 [4] using a semiconductor distributed feedback laser source. A fibre Bragg grating filter of preferably 7.5 GHz is used to reject out-of-band chirped light and, thereby, improve the coherence of the laser pulse sent into the fibre. However, this requires matching of the laser wavelength with the narrow band optical filter, which results in the signal visibility variation being reduced compared to a system which uses a very high coherent source as proposed by U.S. Pat. No. 5,194,847.
Similar techniques have also been proposed for the detection of buried optical fibre telecommunication cables (for example in WO 2004/102840 [7]), in perimeter security (GB 2445364 [8] and US2009/0114386 [9]) and downhole vibration monitoring (WO 2009/056855 [10]). However, the response of these coherent Rayleigh backscatter systems has been limited by a number of parameters such as polarisation and signal fading phenomena; the random variation of the backscatter light; and non-linear coherent Rayleigh response. Therefore these techniques are mainly used for event detection and do not provide quantitative measurements, such as the measurement of acoustic amplitude, frequency and phase over a wide range of frequency and dynamic range.